U.S. patent number 4,268,849 [Application Number 05/957,645] was granted by the patent office on 1981-05-19 for raised bonding pad.
This patent grant is currently assigned to National Semiconductor Corporation. Invention is credited to William M. Gouin, Bruce Gray, James M. Harris.
United States Patent |
4,268,849 |
Gray , et al. |
May 19, 1981 |
Raised bonding pad
Abstract
A bonding pad structure for an LED includes a first layer of
nickel-chromium in contact with the contact face of a gallium
arsenide wafer with further layers of either gold, palladium, or
both, with palladium being the top layer of the first pad structure
laid down by vaporization with a raised structure overlying the
first structure by means of plating and including layers of nickel,
gold and/or other conducting metal.
Inventors: |
Gray; Bruce (Southbury, CT),
Harris; James M. (Palo Alto, CA), Gouin; William M. (San
Jose, CA) |
Assignee: |
National Semiconductor
Corporation (Santa Clara, CA)
|
Family
ID: |
25499905 |
Appl.
No.: |
05/957,645 |
Filed: |
November 3, 1978 |
Current U.S.
Class: |
257/745; 257/737;
257/E23.021; 257/E21.172; 257/766 |
Current CPC
Class: |
H01L
24/13 (20130101); H01L 24/05 (20130101); H01L
24/10 (20130101); H01L 21/28575 (20130101); H01L
33/40 (20130101); H01L 2924/01024 (20130101); H01L
2224/05144 (20130101); H01L 2924/01023 (20130101); H01L
2924/01005 (20130101); H01L 2924/01078 (20130101); H01L
2224/13 (20130101); H01L 2224/13007 (20130101); H01L
2924/01047 (20130101); H01L 2924/01079 (20130101); H01L
2224/05155 (20130101); H01L 2224/13099 (20130101); H01L
2924/01046 (20130101); H01L 2924/01029 (20130101); H01L
2224/05664 (20130101); H01L 2924/12041 (20130101); H01L
2924/01082 (20130101); H01L 2924/01006 (20130101); H01L
2924/01033 (20130101); H01L 2924/10329 (20130101); H01L
2924/01013 (20130101); H01L 2924/014 (20130101); H01L
2224/13 (20130101); H01L 2924/00 (20130101); H01L
2224/05664 (20130101); H01L 2924/00014 (20130101); H01L
2224/05144 (20130101); H01L 2924/00014 (20130101); H01L
2224/05155 (20130101); H01L 2924/00014 (20130101); H01L
2224/05155 (20130101); H01L 2924/01024 (20130101); H01L
2924/013 (20130101) |
Current International
Class: |
H01L
33/00 (20060101); H01L 21/02 (20060101); H01L
23/485 (20060101); H01L 21/285 (20060101); H01L
23/48 (20060101); H01L 023/48 () |
Field of
Search: |
;357/51,61,71 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wojciechowicz; Edward J.
Attorney, Agent or Firm: Brown & Martin
Claims
Having described our invention, we now claim:
1. A bonding pad adapted for wafers of III-V compounds comprising
in combination:
a first layer of metal contact material consisting essentially of a
nickel-chromium alloy bonded to said bonding face, and
a second layer of metal material bonded to said first bonding layer
and consisting of a metal selected from the group consisting of
gold and palladium.
2. The bonding pad of claim 1, comprising:
at least a third layer consisting essentially of a metal selected
from the group consisting of nickel, gold and copper.
3. The bonding pad of claim 2, wherein said first and said second
layers are formed by evaporation, and
said third layer is formed by plating.
4. The bonding pad of claim 1, wherein the first layer is of a
thickness within the range from 500 to 2000 angstroms,
and the second layer is of a thickness within the range from 100 to
2000 angstroms.
5. The bonding pad of claim 4, comprising at least a layer of gold
having a thickness within the range from 500 to 2000 angstroms.
6. The bonding pad of claim 2, comprising a first pad structure
consisting of a plurality of metal layers formed by evaporation,
and
a second pad structure consisting of at least one metal layer
formed by plating a metal selected from the group consisting of
nickel, gold and copper.
7. The bonding pad of claim 6, wherein said first pad structure
comprises a first layer of a nickel chrome alloy, a second layer of
gold, and a third layer of palladium.
8. The bonding pad of claim 7, wherein said second pad structure
consists of a first layer of nickel and a second layer of gold.
9. The bonding pad of claim 7, wherein said second pad structure
consists of a layer of copper.
10. The bonding pad of claim 6, wherein said first pad structure
consists of:
a first layer of nickel-chromium alloy,
a second layer of palladium, and
a third layer of gold.
11. The bonding pad of claim 10, wherein said second pad structure
consists of:
a first layer of nickel,
a second layer of copper, and
a third layer of gold.
12. The bonding pad of claim 11, wherein said second pad structure
consists of:
a first layer of copper, and
a second layer of gold.
13. The bonding pad of claim 7, wherein:
said layer of nickel chrome alloy has a thickness within the range
of 500 to 2000 angstroms,
said layer of gold has a thickness within the range of 500 to 2000
angstroms, and
said layer of palladium has a thickness within the range of 100 to
2000 angstroms.
Description
BACKGROUND OF THE INVENTION
The present invention relates to bonding pad structures and methods
of making same, and pertains particularly to a raised bonding pad
structure for III-V compounds.
Many semiconductor devices are connected into electronic circuits
by means of conductors connected by bonding pads or the like.
Bonding pads and leads must be carefully constructed and selected
to match the characteristics of the elements being placed in the
circuit. The connection of metallic leads to class III-V compounds
ia a particular problem because of the capability of such compound
elements to function at elevated temperatures. The customary
bonding pads suitable for silicon devices has been found not to be
suitable for the group III-V compounds.
Many of the problems of formation of bonding pads to group III-V
compound semiconductors are set out in an article or paper entitled
"A Review of the Theory and Technology for Ohmic Contacts to Group
III-V Compound Semiconductors" by V. L. Rideout, published in the
Solid State Electronics 1975, Volume 18 at pages 541 through 550,
by Pergamon Press, printed in Great Britain. This publication sets
out the background and an introduction to group III-V compound
semiconductors and also points up some of the problems of the
formation of Ohmic contacts for such compound semiconductors.
Various prior art approaches to this problem for such compounds are
also set out in this paper. These, however, have not proven
entirely satisfactory for certain applications. The applicants of
the present invention have devised the present invention to
overcome numerous problems which have not been solved by the prior
art.
The prior art approach to the formation of bonds to bonding pads to
certain group III-V semiconductor compound bodies is also
illustrated in the following U.S. Patents.
U.S. Pat. No. 3,214,654, issued Oct. 26, 1965 to Armstrong et al,
directed to ohmic contacts to III-V semiconductive compound bodies.
This patent sets forth several examples of the fabrication of ohmic
contacts to semiconductor III-V compound bodies. However, he does
not suggest the use of nickel chromium as the first layer of
contact layer. He also does not recognize the advantage of using
palladium as a metalization layer.
U.S. Pat. No. 3,406,050, issued Oct. 15, 1968 to Shortes, is
directed to a method of making electrical contact to a
semiconductor body. This patent discloses an approach to the
formation of bonding pads for semiconductor materials, such as
gallium arsenide. This patent discloses the use of high melting
point refractory metals having a higher melting point than that of
the wafer for attachment to the wafer. However, he does not
disclose the applicant's approach to the formation of ohmic
contacts.
U.S. Pat. No. 3,831,068, issued Aug. 20, 1974 to Kniepknap is
directed to metal semiconductor small surface contacts. In this
patent, contact for gallium arsenide semiconductor device is formed
by a first layer of contact material consisting of chrome with
subsequent layers of nickel, gold and silver. He does not suggest
the applicant's construction.
The present invention is devised specifically for the problems of
group III-V compounds and particularly for LED wafers.
SUMMARY AND OBJECTS OF THE INVENTION
It is accordingly the primary object of the present invention to
overcome the above problems of the prior art.
It is another object of the present invention to provide improved
bonding pad structures for III-V compounds in electrical and
electronic circuits.
A still further object of the invention is to provide an improved
method and bonding pad structure for LED wafers.
In accordance with the primary aspect of the present invention, a
bonding pad for a group III-V compound is formed by first laying
down by evaporation a first layer of nickel-chromium and a second
layer which may be either palladium or gold with the top layer of
the evaporation layers being palladium. Thereafter a raised pad
structure is formed by plating and having a first layer portion of
nickel with further portions selected from the group consisting of
nickel, copper, and gold.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects and advantages of the present invention
will become apparent from the following description when read in
conjunction with the drawings, wherein:
FIG. 1 is an elevational view in section of a preferred pad
structure.
FIG. 2 is a elevational view in section of a wafer structure
prepared for the application of a bonding pad.
FIG. 3 is a view like FIG. 2 showing the first step of forming the
bonding pad.
FIG. 4 is a view like FIG. 2 showing further layers of the pad
structure.
FIG. 5 is a view like FIG. 1 showing an alternate embodiment of the
pad structure.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Turning to the drawings, particularly to FIG. 1, there is
illustrated an exemplary embodiment of pad structure in accordance
with the present invention. The method and apparatus of the present
invention was devised primarily to provide reliable bonding pad
structures for group III-V compounds, particularly that of gallium
arsenide for LED wafer structures.
As best seen in FIG. 1, a wafer 12 of gallium arsenide having an
insulating coating 14 of silicon dioxide or silicon nitride is
provided with a contact face 16 for the connection of a bonding pad
thereto. The bonding pad comprises a first layer of conducting
metal 18 which consists of from 500 to 2000 angstroms of
nickel-chromium. The nickel-chromium alloy layer may have varying
amounts of nickel up to 80% by weight. The nickel-chromium layer 18
is laid down by vaporization in a manner well known in the
industry. This layer provides a good contact system to the gallium
arsenide and acts as a barrier. The next layer of the sub-pad
structure consists of a layer 20 of gold of from approximately 500
to 2000 angstroms thick. This layer is likewise laid down by
vaporization as in the previous layer. The gold coating or layer
provides good conductivity of the pads structure.
A final or top outer coating is provided by a thin layer 22 of
palladium of from 100 to up to 2000 angstroms in thickness. This
final outer metalization coating is also laid down by evaporation.
This coating of palladium provides for ease in subsequent layer
addition by either electrolysis or electrolitic plating.
A second or raised bonding structure is then placed over the first
or lower bonding structure by means of plating. This raised bonding
pad comprises a first pad structure of nickel indicated by the
reference numeral 24 with a upper coating of gold or the like 26.
This FIG. 1 illustration sets forth an exemplary embodiment of the
structure and is preferably laid down in accordance with the steps
described with a thickness within the ranges specified.
The nickel-chromium layer is found to provide metalization that
will adhere to the LED die and be amenable to automatic fabrication
techniques.
The steps of formation of the bonding pads are illustrated in FIGS.
2 through 4 of the drawing. Taking, for example, the LED die 12 as
illustrated, the surface thereof is prepared by having a suitable
passivation protection layer or surface with a hole placed therein
for access to a contact face as illustrated at 16. The passivation
layer may be any suitable material such as silicon dioxide, or
silicon nitride. This LED die or wafer is then formed with a first
series of layers of metal, the first of which is nickel-chromium,
and the subsequent layers may be of the materials as will be
described below, but with the covering layer of palladium; all of
these layers being applied by evaporation or sputtering. This
provides the base or first pad structure. As illustrated in FIG. 3,
up to 3 or more layers may be provided by the evaporation
technique.
Thereafter, a raised bonding pad is then formed on the top of the
first or lower pad structure by means of plating. The first layer
of the plated portion of the illustrated embodiment is a nickel
layer 24. Finally, a layer of gold is plated on the nickel for
proper conductivity.
Turning to FIG. 5 of the drawing, another embodiment is illustrated
wherein an LED wafer 30, similar to that of the previous
embodiment, is selected having a suitable insulation layer 32 with
a contact face 34. A first layer 36 of nichrome of a selected
thickness is evaporated onto the LED wafer with contact properly
made with the contact surface 34. Thereafter, a metalization layer
of palladium 38 is applied as by evaporation. This completes the
base or first portion of the structure.
Thereafter, a further raised bonding pad is provided with a first
layer 40 of nickel or other suitable compound as will be described
below is applied by plating. A coating of a high conductive
material 42 is then layered over the nickel by plating. This second
layer is selected from the group consisting for example, of copper
or alloys or nickel gold, copper gold, nickel copper, or nickel
copper and gold. The heart of the invention is the use of
nickel-chromium as a contact system directly to the gallium
arsenide and palladium as the metalization layer which allows for
subsequent plating by electrolysis imersion of electrolytic for
further contact construction.
By way of example, the bonding pad contacts of the present
invention may be formed in a number of alternate constructions.
These are set forth in the following examples.
EXAMPLE 1
The structure of the system may consist of a gallium arsenide chip
of wafer on which is placed a nickel chromium content layer at the
contact face followed by a gold layer and then a palladium layer.
These three layers are deposited in the manner as described above,
such as evaporation, and/or sputtering. Thereafter, further layers
of contact metal are placed thereon which metals are selected from
the group consisting of gold, nickel gold, copper, copper gold,
nickel copper, or nickel copper gold. One or more contact layers of
these metals are placed over the basic pad structure by means of
plating or the like. This is carried out in the usual manner of
forming raised bonding pads or bumps on bonding pads.
EXAMPLE 2
The wafer is prepared in the manner as described above and
thereafter a series of layers of the following metals are
evaporated onto the contact face of the wafer. These metals, in
order, consist essentially of nickel chromium followed by a layer
of palladium followed by a layer of gold. Thereafter, contact
materials of nickel followed by a layer of copper and a layer of
gold are plated onto the base pad structure forming the raised
bonding bump or pad structure.
EXAMPLE 3
In the same method as described above, a wafer is prepared and the
following metals evaporated thereon. These metals consist
essentially of nickel chrome alloy followed by a layer of gold,
followed by a layer of palladium, which in turn is followed by a
layer of gold. These layers are laid down by evaporation or
sputtering, preferably evaporation. Thereafter, a layer of copper
is plated onto the pad structure forming the raised bonding pad or
bump.
EXAMPLE 4
The methods as described above are used in the preparation of a
wafer on which is placed a pad as follows. A first layer of nickel
chrome alloy is evaporated onto the contact face followed by a
layer of palladium, followed by a layer of gold. Thereafter, layers
of copper followed by a layer of gold are plated onto the pad
forming the raised bonding pad.
EXAMPLE 5
The pad structure of the invention is adapted to wire (leads)
bonding by eliminating the raised structure in either FIGS. 1 or 5
and bonding wire leads directly to the base pad. Thus, a wire lead
can be bonded directly to a pad of nickel-chrom and gold or
nickel-chrome and palladium-gold.
By this method an improved pad structure is formed which is found
to overcome many of the problems associated with the prior art
approaches to the fabrication of pads for devices of this
character. The present invention was developed and devised
specifically to overcome the problems associated with the forming
of bonding pads for light emitting diodes formed from gallium
arsenide.
While we have illustrated and described our invention by means of
specific examples and embodiments, it is to be understood that
numerous changes and modifications may be made therein without
departing from the spirit and scope of the invention as defined in
the appended claims.
* * * * *